1. Field of the Invention
This invention relates to cross-linked polyvinyl alcohols and to the use thereof in controlling the permeability of subterranean oil-bearing formations.
2. Discussion of Related Art
In the production of oil from subterranean oil reservoirs by various flooding techniques, especially waterflooding, it has become a common expedient to add various polymeric thickening agents to the flooding liquid, such as water, to decrease its mobility to a point where it approaches that of the crude oil which is to be displaced so as to improve the displacement of the oil from the reservoir. The use of polymers for this purpose is often stated to be for mobility control.
Another problem which arises in the various flooding processes is that different strata or zones in the reservoir often possess different permeabilities so that displacing fluids enter the high permeability or "thief" zones in preference to zones of lower permeability where significant quantities of oil may be left unless measures are taken to at least partially plug the high permeability zones to divert the displacing fluid into the low permeability zones. Mechanical isolation of the thief zones has been tried but vertical communication among reservoir strata often renders such measures ineffective. Physical plugging of the high permeability zones by cements and solid slurries has also been attempted with varying degrees of success but the most serious drawback of this approach is the possibility of permanently closing productive portions of the reservoir.
From these early experiences, the desirability of designing a viscous slug capable of sealing off the most permeable layers so that the floodwater or other driving fluid, such as gas or steam, would be diverted to the underswept, tighter regions of the reservoir, became evident. This led to the use of oil/water emulsions, gels and polymers for controlling the permeability of the formations in a process frequently called "profile control" or "flood conformance", a reference to control of the vertical permeability profile of the reservoir. Profile control agents which have been proposed include oil/water emulsions, gels, e.g., lignosulfonate gels and polymers, with polymers being the most extensively used in recent years.
Among the polymers so far examined for improving flood conformance are polyacrylamides, polysaccharides, celluloses, furfural-alcohol and acrylic/epoxy resins, silicates and polyisocyanurates. A major part of this work has been conducted with the polyacrylamides, both in their normal, non-crosslinked form, as well as in the form of metal complexes, as described, for example, in U.S. Pat Nos. 4,009,755; 4,069,869 and 4,413,680. In either form, the beneficial effects derived from these polyacrylamides seem to dissipate rapidly due to shear degradation during injection and sensitivity to reservoir brines, low pH and high temperature. To overcome these problems and to achieve deeper polymer penetration into the reservoir, dilute solutions of these polymers have sometimes been injected first and then complexed in situ.
Another group of polymeric thickeners which has received considerable attention for use in improving flooding are polysaccharides, particularly those produced by the action of bacteria of the genus Xanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and 3,383,307 disclose a process for mobility control by the use of polysaccharides.
U.S. Pat. Nos. 3,741,307; 4,009,755; 4,069,869 disclose the use of polysaccharides in the control of reservoir permeability. U.S. Pat. No. 4,413,680 describes the use of cross-linked polysaccharides for selective permeability control in oil reservoirs.
U.S. Pat. No. 3,908,760 describes a polymer waterflooding process in which a gelled, water-soluble Xanthomonas polysaccharide is injected into a stratified reservoir to form a slug, band or front of gel extending vertically across both high permeability and low permeability strata. This patent also suggests the use of complexed polysaccharides to block natural or man-made fractures in formations. The use of polyvalent metal ions for cross-linking polysaccharides is also disclosed in U.S. Pat. No. 3,810,882.
The use of various block copolymers for mobility control in waterflooding operations is described in U.S. Pat. Nos. 4,110,232, 4,120,801 and 4,222,881, but their use for permeability control has not been suggested.
Chung et al, U.S. Pat. No. 4,653,585, disclose the use of block copolymers, which may be cross-linked with polyvalent metal ions, as permeability control agents in enhanced oil recovery applications.
While a number of different compositions, such as polymers discussed above, have been proposed for permeability control, some of such agents may be unsuitable for use as permeability control agents under certain circumstances. For example, the polymers of Chung et al, may not be effectively cross-linked with polyvalent metal ions under all conditions encountered in the enhanced oil recovery applications, e.g., in acidic conditions commonly found in carbon dioxide (CO.sub.2) flooding operations. Similarly, some polymers are unsuitable as permeability control agents used in conjunction with steam flooding operations because they lose their structural integrity (i.e., they undergo "syneresis") at the high temperatures generated during such operations. This has led to the development of so-called hostile environment (HE) polymers, such as those marketed by Phillips Petroleum and Hoechst. However, a need still exists in the art for permeability control agents which can be used for such applications in diverse reservoir conditions, such as steam flooding.
Accordingly, it is a primary object of the present invention to provide a composition of matter which is a cross-linked polyvinyl alcohol.
It is another object of the invention to provide a polymer gel which can be used effectively as a permeability control agent under the extreme temperature conditions encountered in steam flooding of underground formations.
Other objects of the invention will become apparent to those skilled in the art from the following description of the invention.